The purpose of my thesis is to defend what I take to be Kant's internalist theory of moral obligation. I argue that any defense of Kant on this score, requires a defense of Kant's theory of freedom, as well as the Kantian claim that reason can function as a motive cause of behaviour. In defense of this claim, I discuss the development of British moral philosophy from Hobbes through Hume. Focusing primarily on the Empiricists, I argue that a commitment to Newtonian mechanism led many to embrace a reductivist theory of human psychology and a naturalistic explanation of human motivation. As a result, many British moral philosophers adopted a hedonistic theory of action. I argue that Kant's criticism of the British Empiricists on these and other related issues, provides an important backdrop against which Kant's own theory of obligation emerged. In particular, Kant rejected reductivist accounts of the mental and physical since any claims to know the nature of 'things in themselves' oversteps the bounds of Reason. Thus, freed from a commitment to mechanism, Kant did not have to accept a compatibilist theory of freedom in order to explain moral agency. I claim Kant's theory of freedom is metaphysically committed to a non-reductive, token-token identity theory where 'intelligible' causal descriptions are not reducible to 'empirical' causal descriptions. As such, Kant can account for the causal efficacy of reason since he can allow for the spontaneity of thought freed from desire. In order to demonstrate the connection between Kant's theory of action and his conception of obligation, I then discuss Kant's claim that persons must always be treated as ends and never simply as means. I argue that Kant's conception of persons as finite, autonomous, rational creatures informs the principle of reason that obligates us to respect others. Therefore, the principle of obligation is internal to reason itself. And, since we are autonomous rational creatures, we are capable of adopting and acting on principle. Thus, obligation and our ability to act according to the dictates of reason go hand in hand.